1. Field of the Invention
The present invention relates to an in-mold foam molding apparatus and method suitable for molding into a unitary molding molded portions comprising bead starting materials having different properties, and to in-mold foam molded articles.
2. Description of the Related Art
An in-mold foam molding apparatus for fabricating moldings from bead starting materials consisting of thermoplastic synthetic resin is taught, for example, in U.S. Pat. No. 5,164,257, which discloses an in-mold foam molding technique wherein the mold is provided with moveable partitioning members that are retractable from the mold cavity via actuators such as air cylinders, the mold cavity being partitioned into a plurality of partitioned mold chambers by means of these moveable partitioning members, with filling devices for supplying bead starting materials to the individual partitioned mold chambers being connected therewith individually, whereby with the mold cavity partitioned by the moveable partitioning members, adjacent partitioned mold chambers can be filled with bead starting materials having, for example, different degrees of expansion, and once so filled the moveable partitioning members can be retracted and steam supplied to the interior of the mold cavity in order to heat and weld the bead starting materials into a molded article.
In molded articles molded by means of this in-mold foam molding technique, by varying the bead starting materials used for different regions of a molded article it is possible to fabricate a molded article having, for example, different mechanical properties in different regions thereof. This has the advantage of being able to improve functionality and quality in molded articles, which are used as cores for car bumpers and cushioning materials for packaging of household electronics, furniture, and the like.
In the in-mold foam molding apparatus disclosed in the cited US publication, it is necessary to provide actuators for retracting the partitioning members, and thus a larger number of partitioned mold chambers will require a correspondingly complicated drive system for the partitioning members, resulting in the problem of higher fabrication costs for the in-mold foam molding apparatus.
Further, if filling compression in adjacent partitioned mold chambers is not controlled in such a way as prevent pressure differentials from forming, there is a risk of a partitioning member deforming due to the pressure differential. Another problem is that the flow of air used for filling is obstructed by the partitioning members, depressing ease with which the mold may be filled with the bead starting materials.
In this in-mold foam molding apparatus, since passage orifices are formed in the mold so that the partitioning members may be retracted through these passage orifices, there is the additional problem that flash forms on the surface of the molded article around these passage orifices for the following two reasons.
(1) Reason 1
In order to prevent the bead starting materials from infiltrating, the wall of the passage orifice and the partitioning member will ideally be designed with as narrow a gap as possible therebetween, but this arrangement creates the problem of deformation or breakage of a partitioning member or inability to retract a partitioning member due to contact with the inside wall of a passage orifice resulting from expansion or contraction of the mold. Specifically, the mold expands when the bead starting materials are steam heated and contracts during cooling with cooling water. Where passage orifices like those described earlier are provided, mold strength is lower in the portions bordering the passage orifices, so strain in the mold becomes concentrated at the passage orifices, and is compensated for by expansion and contraction of the width of the apertures of the passage orifices. Where the passage orifices are of “L” or “C” configuration, lower mold strength in a specific direction will result in shape deformation of passage orifices. Expansion/contraction or deformation of passage orifices may result in deformation or breakage of a partitioning member through contact with the inside wall of a passage orifice. Where the mold is split into a plurality of molding sections by the passage orifices with the plurality of molding sections being fixed by means of screws or the like to an attachment plate, it will be a simple matter to form passage orifices in the mold, but expansion or contraction of the mold will result in shifting of the locations at which the mold segments are attached to the attachment plate, so that there will be significant deviation in the width of the passage orifice apertures. This phenomenon becomes more pronounced with larger mold dimensions or longer passage orifices, and represents a significant problem for in-mold foam molding apparatuses equipped with partitioning members. For such reasons, the partitioning member and the wall of the passage orifice are designed with a large gap therebetween, notwithstanding the fact that it is recognized that flash will form on the surface of the molded article around the passage orifices.
(2) Reason 2
Partitioning member distal edge shape is designed to conform to the shape of the inside surface of the mold, but as shape is not always the same as that of the inside surface of a mold provided with passage orifices, it may occur that when a partitioning member is retracted after filling the mold with bead starting materials, the partitioning member recesses partway into its passage orifice so that the interior of the passage orifice communicates with the mold cavity. Since the width of the passage orifice apertures is designed to be smaller than the diameter of the bead starting materials in order to prevent bead starting materials from infiltrating into passage orifices, merely retracting the partitioning members will not result in bead starting materials infiltrating into passage orifices, but when the bead starting materials are heated and welded by delivering steam to the mold cavity, softening and expansion of the bead starting materials may result in partial infiltration into passage orifices, producing long, thin flash projecting out at locations on the molded article corresponding to the locations of passage orifices.
Flash is not a particular problem for molded articles of which only rough dimensional accuracy on the surface is required, but represents a product defect for molded articles subject to strict requirements. For example, in car bumpers, it is common practice to affix an in-mold foam molded core to the front face of front beam of the car, and to then arrange a synthetic resin cover member so as to cover the core. With car bumpers of this type, flash must be removed from the core after molding in order to prevent problems such as inability to secure the core to the proper location on the mounting face of the front beam or inability to secure the cover member at the proper location on the vehicle body due to the presence of a gap between core and front beam or core and cover member.
Japanese Unexamined Patent Application H10-193375 discloses a molding apparatus for molding foam molded articles that have molded sections consisting of bead starting materials of different properties and that can be used as shock absorbent materials, the apparatus provided with fixed partitioning members situated at the boundaries of adjacent partitioned mold chambers, with these partitioning members defining spaces wherein the bead starting materials are fused. This molding apparatus is designed such that slits are formed in the molded article by the partitioning members, whereby the molded article can be split at the slits into smaller pieces by hand, thereby facilitating disposal or recycling of the molded article after use. However, the design of the molding apparatus disclosed in this publication is provided with these partitioning members situated along certain portions of the boundaries between adjacent partitioned mold chambers, in order to form slits in the molded article, and while the specifics of the partitioning arrangement for the other portions of the boundaries is not disclosed, it may be surmised that partitioning thereof is accomplished by means of partitioning members that are retractable from the mold, since fusion of the bead starting materials overall can be visually confirmed.
An in-mold foam molding apparatus of typical configuration for producing molded article from bead starting materials comprising thermoplastic synthetic resin is depicted in FIG. 35, wherein a set of opposing molds 200, 201 is provided, the two molds 200, 201 having chambers 202, 203 formed on their respective back faces and the two molds 200, 201 having respectively formed therein a multitude of air orifices 205, 206 whereby chambers 202, 203 communicate with a mold cavity 204 so that a service fluid, such as steam, may be delivered to molding cavity 204 via air orifices 205, 206 or vented from molding cavity 204. In this example, respective chambers 202, 203 are provided in the top portions thereof with top service orifices 207, 208 for delivering as heating steam or the like, and in the bottom portions thereof with bottom service orifices 209, 210 connecting to a vacuum pump or drain pipe so that steam may delivered to molding cavity 204.
In actual practice, as shown in FIGS. 36 and 37, the multitude of air orifices 205, 206 extending through molds 200, 201 are composed of core vents 211—capped tubular elements having an outside diameter of 7–12 mm and perforated by a plurality of air orifices 205, 206 comprising round orifices about 0.5 mm in diameter or slits about 0.5 mm wide—which fit within core vent mounting orifices 212 provided to molds 200, 201; and core vent holes 213 about 0.5 mm in diameter formed directly in molds 200, 201. Air orifices 205, 206 are arranged at 20–50 mm pitch on molds 200, 201.
In an in-mold foam molding apparatus of this kind, pre-expanded bead starting materials are packed into mold cavity 204, heated with steam to bring about expansion and fusion thereof, and then cooled and hardened to give a foam molded article of the desired configuration. The function of air orifices 205, 206 during foam molding is discussed further.
Japanese Unexamined Patent Application S57-174223 discloses a process diagram like that shown in FIG. 38 wherein (a) to (e) depict a preheating/evacuation process for replacing air in the mold and air between bead starting materials with steam, the specifics being described hereinbelow. In the drawing, solid black valve symbols indicate the closed state and white valve symbols indicate the open state.
(a) is an evacuation step wherein once the bead starting materials have been packed into the mold cavity 204 steam is delivered for a very brief period to chambers 202, 203 via top service orifices 207, 208, and air in the mold interior, and particularly in chambers 202, 203, is suctioned and evacuated therefrom via bottom service orifices 209, 210 to evacuate the interior. Here, chambers 202, 203 are brought to positive pressure by the steam, and steam penetrates between the bead starting materials via air orifices 205, 206.
(b) is a two-end evacuation step wherein top service orifices 207, 208 are closed while continuing the suction evacuation procedure to lower the pressure within the mold, whereby any air present between the bead starting materials is suctioned and evacuated via air orifices 205, 206 provided to both ends of the mold.
(c) is a one-sided preheating step wherein bottom service orifices 209, 210 are closed and steam is delivered for a brief period via the top service orifice 208 of one of the depressurized chambers 203. The supplied steam flows from air orifice 206 of mold 201, through the bead starting materials in mold cavity 204, and through air orifice 205 of mold 200 to reach chamber 202 on the opposite side, thereby heating the bead starting materials and all areas of molds 200, 201.
(d) is a one-sided preheating step wherein the direction of steam flow is reversed, wherein an analogous procedure is conducted, but from the chamber 202 side, whereby air present within mold cavity 204 is completely expelled and preheating is conducted while minimizing localized temperature differentials between the two molds 200, 201.
(e) is a fusing/heating step wherein steam for fusing/heating is supplied to both chambers 202, 203, thereby heating the molds 200, 201 and also heating the bead starting materials via the air orifices 205, 206 of the respective molds 200, 201 in order to complete expansion and fuse the beads together to produce a foam molded article.
The air orifices 205, 206 provided to molds 200, 201 function as passages for evacuating air present between the bead starting materials and as passages for delivering steam, and as such serve an important function in terms of producing a homogeneous foam molded article. On the other hand, the following problems have been noted.
(1) To compensate for lower mold strength resulting from the mold being perforated by a multitude of air orifices, mold wall thickness in molds consisting of aluminum alloys must be on the order of 8–12 mm, for example. However, this has the effect of increasing the heat capacity of the mold, lowering heat efficiency during heating/cooling so that the rate of temperature rise and temperature drop are slower, reducing the precision of control.
(2) Typically, a pair of molds is provided with some 2000 to 4000 air orifices, so the process of making the orifices is complicated and results in higher fabrication costs. Since the core vents are installed by hand in mounting orifices provided in the mold, the operation is quite complicated and damage to mold surfaces is unavoidable, thus requiring retouching.
(3) Since clogging of air orifices (core vents, core vent holes, etc.) by scale or the like can result in heating defects, mold release defects, or cooling defects, the core vents must be replaced or periodically subjected to high pressure washing or other maintenance procedure.
(4) Since air orifices leave marks on foam molded article surfaces, the visual appeal of molded articles suffers, and when the exterior surface is subjected to printing or the like, air orifice marks may impair printing.
(5) Since the foam molded article is cooled by spraying cooling water into the chamber after molding, water infiltrates into the molding cavity through the air orifices, resulting in water content of about 6–10% in the molded article, necessitating a drying process. Further, since cooling water comes into direct contact with the molded article, pure cooling water must be used in order to produce sanitary molded articles.
(6) As steam is passed from the chamber into the mold cavity to heat the bead starting materials under the same heating conditions in order to effect expansion/fusion thereof, molded articles produced in this way (hereinbelow referred to as isothermal molded articles) develop varying surface qualities depending upon the extent of fusion of the beads. Specifically, lower fusion rates are associated with poor surface qualities, whereas higher fusion rates associated with good surface qualities. For isothermal molded articles, higher bead fusion rates improve physical properties such as the mechanical strength of the molded article, but require longer heating, expansion/fusion times and cooling times, creating the problem of longer molding cycle times overall and reduced throughput.
For such reasons, in the molding technique described earlier, bead fusion rates are typically set to 40%–80%, for example, in order to assure good surface qualities and attractive appearance as well as assuring a fusion rate adequate to assure mechanical strength. However, even where mechanical strength requirements for a molded article are not particularly stringent, the need to assure an attractive appearance requires a moderately high fusion rate, which will result in a correspondingly longer molding cycle time and reduced throughput. Fusion rate as used herein is ascertained by splitting the molded article and observing the condition of the beads on the sectional face, specifically, by measuring the proportion of beads experiencing breakdown of the bead per se, deeming beads having cracking along the bead surface but without bursting of the bead per se to be unfused and deeming beads experiencing bursting of the bead per se into fragments to be fused.
The foam molding process described hereinabove is designed such that air orifices such as core vents and core vent holes are used to deliver steam, air, or other service fluids to the mold cavity or to evacuate same from the mold cavity during production of foam molded articles. However, as noted, the provision of air orifices creates a number of problems.
With the goal of providing a fundamental solution to these problems, the inventors conducted extensive research concerning development of a foam molding process employing molds devoid of air orifices, and conducted tests of various kinds. While the goal is a mold “devoid of air orifices,” it is of course necessary to provide, in lieu of core vents and core vent holes, passages for delivering/evacuating steam, air, or other service fluids to and from the mold cavity, which gives rise to the issue of where and how to form same, of the timing and conditions that should be employed in delivering service fluids to such passages, and a host of other issues that need to be addressed.